The present invention relates to an electric-light bulb, more precisely relates to an electric-light bulb, which is capable of automatically adjusting an electric current passing through a filament.
A conventional electric-light bulb is shown in FIGS. 7A and 7B. A base 52 is provided to a glass bulb body 54. A pair of lead lines 56a and 56b are provided in the bulb body 54 and vertically extended from a pitch 58 with a separation. A filament 60 is spanned between upper ends of the lead lines 56a and 56b. The base 52 acts as terminals for supplying an electric current to the lead lines 56a and 56b.
When the electric current is supplied to the lead lines 56a and 56b via the base 52, the filament 60 is red-heated by the electric current supplied and radiates light, so that the electric-light bulb 50 is turned on.
In the room temperature, resistance of the filament 60 is low, so an inrush current, which is 13-16 times as great as a rated current, instantaneously passes through the filament 60 when the electric-light bulb 50 is turned on. By turning on and off the electric-light bulb 50, a great load is applied to the filament 60, so that a span of life of the electric-light bulb 50 must be shorter.
In the case of an electric-light bulb which is used at a place where the electric-light bulb cannot be easily exchanged, etc. and which must have a long span of life, a resistance (not shown) is provided in a bulb body and connected to a filament in series so as to limit intensity of electric current passing through the filament. Further, a thermistor, whose resistance is reduced when temperature of the filament rises, is provided so as to limit the inrush current passing through the filament.
The conventional electric-light bulbs, which include various types of bulbs in which filaments radiate light, are used for decorating show windows, Christmas trees, etc. and calling people's attention at construction sites, etc. Further, in some cases, a plurality of electric-light bulbs are connected and automatically turned on and off.
To automatically turned on and off the electric-light bulb, a bimetal element is provided in a bulb body and connected to a filament in series. The bimetal elements turns on and off the electric-light bulb as a thermoswitch.
In an initial state, temperature in the bulb body is low, so contact points of the thermoswitch are closed by the bimetal element. An electric current can be supplied to the filament.
When the filament radiates heat and temperature of the bimetal element reaches prescribed temperature, the contact points are opened, so that the electric current passing through the filament is stopped.
By stopping the current supply, the temperature in the bulb body gradually goes down, then the contact points are closed again and the electric current can be supplied to the filament again.
By repeating above described actions, the electric-light bulb can be automatically turned on and off.
However, the conventional electric-light bulb has following disadvantages.
Firstly, in the case of the electric-light bulb having the thermistor for limiting the inrush current, manufacturing cost must be increased because the thermistor is expensive.
Secondly, in the case of using the electric-light bulbs for decorating windows, etc. and calling attention, the conventional electric-light bulbs can be merely turned on and off. These days, electric-light bulbs, which is capable of changing brightness, are required so as to more effectively decorating or calling attention. But the conventional electric-light bulbs cannot automatically change their brightness.